UKBA is still dealing with the scandal that emerged in 2006, when half a million asylum applications were discovered gathering dust in boxes at the Home Office, and has so far been unable to contact a fifth of the individuals in the backlog.

There are almost 4,000 foreign criminals at large who should have been considered for deportation when they were released from jail.

The Home Office is to pay a private company, Capita, up to £40million if it manages to contact the 150,000 people whose visas to work or study in Britain have run out.

Border officials also rely on tip-offs from the public to help find illegal immigrants, and recent figures show that some 20,000 allegations are made every three months.

Although almost all are assessed within 48 hours and half are investigated properly by officials, only about 600 lead to arrests.

Until now, information about allegations has been held on a separate list to detail about who is removed from the country.

But as of this weekend, the National Allegations Database will assign a unique reference number to cases across both systems so that the UKBA can track the outcome of all allegations.

The database will provide a more centralised approach as allegations were previously recorded by hand with information kept in local offices.

Early next year the online form for reporting tip-offs will be redesigned, in order to give informants a better idea of what details they need to provide about suspected illegal immigrants.

The Home Affairs Select Committee has called for people who make genuine allegations to be told about the outcome, to increase confidence in the system.